This invention relates to a method for removing impurities and contaminants from caprolactam and, in particular, to a method for removing compounds having hydroxy, amino or carboxyl functional groups from caprolactam.
The production of yarns from polycaproamide, or, more commonly, nylon-6, requires the utilization of .epsilon.-caprolactam of substantial purity. Experience has shown that the use of impure lactam often will lead to poor polymerizations and will produce yarn which is, variously, of poor color, brittle, or which decomposes or degrades upon storage. Additionally, impure lactam itself often has an off color, which is not favored in the nylon industry.
While it is, of course, possible for caprolactam to become contaminated with any of a vast number of possible impurities, certain types of contamination are encountered with sufficient frequency to be of commercial concern.
Of foremost concern as a contaminant is ethylene glycol, which may be introduced into lactam in a variety of ways. For example, it is the usual practice to depolymerize waste nylon in order to recover caprolactam, which is rather costly, and to the reuse the recovered lactam as a feedstock for further nylon production. If, as sometimes happens, ethylene glycol derived polyester waste becomes commingled with the nylon, either by accident or design, the resulting stream of depolymerized waste will contain not only caprolactam but also ethylene glycol and terephthalic acid as a result of the depolymerization of the polyester.
Similarly, it is a usual practice to clean nylon 6 polymerization vessels by boiling them with ethylene glycol, which serves to depolymerize deposits in the vessel interior. Failure to adequately purge the glycol so introduced will, of course, cause any nylon produced in the vessel to be contaminated with ethylene glycol. As explained before, the nylon so produced will be subject to various defects. But, further, it is a standard practice to wash chips of freshly polymerized nylon in water in order to recover unreacted lactam and oligomers. Experience has shown that lactam recovered from glycol contaminated nylon chips will itself be contaminated with ethylene glycol. It is easy to see that if this contaminated lactam is then reused as a feedstock, a nylon-6 production line can become chronically contaminated with glycol and that the nylon produced by the line will be of an inferior quality.
Other contaminants may also find their way into caprolactam, although their occurrence may be less common. Thus, contamination by alcohols other than ethylene glycol may occur if lactam is recovered through the depolymerization of waste nylon-6 which has been mixed, either purposefully or inadvertently, with polyester derived from such other alcohols, e.g., propylene glycol in the case of polypropylene terephthalate, or butylene glycol in the case of polybutylene terephthalate. And, of course, terephthalic acid contamination will also result from the depolymerization of these polyesters, as in the case of the depolymerization of polyethylene terephthalate polyester. Contamination by compounds having both amino and carboxyl functional groups may occur, for example, if nylon-6 waste is mixed with nylon-66 and then depolymerized, the depolymerization of nylon-66 yielding hexamethylenediamine and adipic acid.
Although prior inventors have presented methods for removing various contaminants from caprolactam, most notably alkalene glycols such as ethylene glycol, it is believed that none of these prior methods is entirely satisfactory.
Accordingly, it is a general object of this invention to provide a method for removing impurities from caprolactam so as to yield a product of exceptional purity.
It is a more specific object to provide a method for removing impurities from caprolactam which fall within the class of compounds comprising hydroxy, amino or carboxyl functional groups.
A still more specific object is to provide a method for removing alkalene glycols, particularly ethylene glycol, from caprolactam.
Another specific object of the invention is to provide a method for removing color causing impurities from caprolactam.
And, as further objects, the invention is intended to provide a method for removing impurities from caprolactam which is both suitable for use on a large scale and economical.